Hartley oscillator circuit theory working and application

Hartley Oscillator Circuit
Theory Working and
Application

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Introduction:
Hartley Oscillator Circuit Theory
Working and Application
 The Hartley oscillator is an electronic oscillator circuit in which the oscillation
frequency is determined by the tuned circuit consisting of capacitors and
inductors, that is, an LC oscillator. The Hartley oscillator was invented by
Hartley while he was working in the Research Laboratory of the Western
Electric Company. The circuit was invented in 1915 by American engineer
Ralph Hartley.

What are Hartley Oscillators?
 Hartley oscillator is inductively coupled.
 It's a variable frequency oscillators where the oscillator may be a
series or shunt fed.
 Hartley oscillators is the advantage of having one tuning capacitor
and one center tapped inductor.
 This processor simplifies the construction of a Hartley oscillator
circuit.

What are Hartley Oscillators?
Hartley Oscillator

Hartley Oscillator Circuit and Working
 The circuit diagram of a Hartley oscillator consists of an NPN transistor
connected in a common emitter configuration.
 It works as the active device in amplifier stage.
 R1 and R2 are biasing resistors.
 CC1 and CC2 are the coupling capacitors
 L1 and L2 are Inductance.

 RFC is the radio frequency choke.
 Which provides the isolation between AC and DC operation.
 At high frequencies, the reactance value of this choke is very high.
 So, it can be treated as an open circuit.

 The reactance is zero for DC condition, hence causes no problem for DC
capacitors.
 The CE is the emitter bypass capacitor.
 RE is also be a biasing resistor.
 The CC1 and CC2 are the coupling capacitors.

 When the DC supply (Vcc) is given to the circuit.
 The collector current starts raising and begins with the charging of the
capacitor C.
 Once capacitor C is fully charged, it starts discharging through L1 and L2
and again starts charging.
 This back-and-fourth voltage waveform is a sine wave which is a small
and leads with its negative alteration.
 It will eventually die out unless it is amplified.

 Now the transistor comes into the picture.
 The sine wave generated by the tank circuit is coupled to the base of
the transistor through the capacitor CC1.
 Since the transistor is configured as common-emitter,
 It takes the input from tank circuit and inverts it to a standard sine
wave with a leading positive alteration.

 The transistor provides amplification along with inversion to
amplify and correct the signal generated by the tank circuit.
 The mutual inductance between L1 and L2 provides the feedback
of energy from collector-emitter circuit to the base-emitter
circuit.
 The frequency of oscillations in this circuit is
fo = 1/ (2π √ (Leq C))

 Where Leq is the total inductance of coils in the tank circuit is given as
Leq = L1 + L2 + 2M
 For a practical circuit, if L1 = L2 = L and the mutual inductance are
neglected, then the frequency of oscillations can be simplified as
fo = 1/ (2π √ (2 L C))

Hartley Oscillator Circuit Using Op-Amp
 The Hartley oscillator can be implemented by using an operational
amplifier.
 This type of circuit facilitates the gain adjustment by using feedback
resistance and input resistance.

 In transistorized Hartley oscillator, the gain depending up on the tank
circuit elements like L1 and L2.
 Whereas in Op-amp oscillator gain is less depends on the tank circuit
element.
 Hence provides greater frequency stability.
 The operation of this circuit is similar to the transistor version of Hartley
oscillator.

 The sine wave is generated by the feedback circuit and it’s coupled with
the op-amp section.
 Then this wave is stabilized and inverted by the amplifier.
 The frequency of an oscillator is varied by using a variable capacitor in
the tank circuit.
 Keeping the feedback ratio and the amplitude of the output is constant
for over a frequency range.

 The frequency of oscillations for this type of oscillator is same as
the above discussed oscillator and is given as
fo = 1/ (2π √ (Leq C))
Where: Leq = L1 + L2 + 2M
Or
Leq = L1 + L2

 To generate the oscillation from this circuit, the amplifier gain must
and should selected greater than or at least equal to the ratio of two
inductances.
 If the mutual inductance exists between L1 and L2 because the common
core of these two coils, then the gain becomes
Av = L1 / L2
Av = (L1 + M) / (L2 + M)

Advantages of Hartley Oscillator
 Instead of two separate coils L1 and L2, a single coil of bare wire can be
used and the coil grounded at any desired point along it.
 By using a variable capacitor or by making core movable (varying the
inductance), frequency of oscillations can be varied.
 Very few components are needed, including either two fixed inductors
or a tapped coil.
 The amplitude of the output remains constant over the working
frequency range.

Disadvantages of Hartley Oscillator
 It cannot be used as a low frequency oscillator since the value of
inductors becomes large and the size of the inductors becomes large.
 The harmonic content in the output of this oscillator is very high.
 Hence it is not suitable for the applications which require a pure sine
wave.

Applications
 The Hartley oscillator is to produce a sine wave with the desired
frequency.
 Hartley oscillators are mainly used as radio receivers.
 Due to its wide range of frequencies, it is the most popular oscillator.
 The Hartley oscillator is Suitable for oscillations in RF (Radio-
Frequency) range, up to 30MHZ

Conclusion
 This is all about Hartley oscillator circuit theory working and
applications. The personal feature of Hartley oscillator is that the
tuned circuit consists of a single capacitor in parallel with two
inductors are in series or a single tapped inductor, and the feedback
signal needed for oscillation is taken from the center connection of
the two inductors.

Hartley oscillator circuit theory working and application

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